Field of the Invention
This invention relates generally to a system and method for depleting the oxygen in a fuel cell stack and, more particularly, to a system and method for creating a volume of oxygen depleted gas throughout as much of the cathode sub-system as possible.
Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode electrodes, or catalyst layers, typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, the gas diffusion layer (GDL), that protects the mechanical integrity of the membrane and also helps in uniform reactant humidity diffusion. MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a by-product of the chemical reaction taking place in the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include anode side and cathode side flow distributors, or flow fields, for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Water is generated as a by-product of the stack operation, therefore, the cathode exhaust gas from the stack will typically include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the water transfer elements, such as membranes, is absorbed by the water transfer elements and transferred to the cathode air stream at the other side of the water transfer elements.
When a fuel cell system is shut down, unreacted hydrogen gas remains in the anode side of the fuel cell stack. This hydrogen gas is able to diffuse through or cross over the membrane and react with the oxygen in the cathode side. As the hydrogen gas diffuses to the cathode side, the total pressure on the anode side of the stack is reduced, where it is possible to reduce the pressure below ambient pressure. This pressure differential can draw air from ambient into the anode side of the stack. It is also possible for air to enter the anode by diffusion from the cathode. When the air enters the anode side of the stack it can generate air/hydrogen fronts that creates a short circuit in the anode side, resulting in a lateral flow of hydrogen ions from the hydrogen flooded portion of the anode side to the air-flooded portion of the anode side. This current combined with the high lateral ionic resistance of the membrane produces a significant lateral potential drop (˜0.5 V) across the membrane. This produces a local high potential between the cathode side opposite the air-filled portion of the anode side and adjacent to the electrolyte membrane that drives rapid carbon corrosion, and causes the electrode carbon layer to get thinner. This decreases the support for the catalyst particles, which decreases the performance of the fuel cell.
In automotive applications, there are a large number of start and stop cycles over the life of the vehicle and the life of the fuel cell system each of which may generate an air/hydrogen front as described above. An average vehicle can experience 40,000 startup/shutdown cycles over its useful life. Start and stop cycles are damaging to the fuel cell system due to the potential which may be generated by an air/hydrogen front, and the best demonstrated mitigation of damage still causes approximately 2 to 5 μV of degradation per start and stop cycle. Thus, the total degradation over the 40,000 start and stop cycle events can exceed 100 mV. However, by not allowing air to enter the fuel cell stack while the fuel cell system is shutdown, damage during subsequent restarts may be reduced or prevented.
It is known in the art to purge the hydrogen gas out of the anode side of the fuel cell stack at system shutdown by forcing air from the compressor into the anode side at high pressure. However, the air purge creates the air/hydrogen front discussed above that causes at least some corrosion of the carbon support structure.
Another known method in the art is to provide cathode re-circulation to reduce cathode corrosion at system shutdown. Particularly, it is known to pump a mixture of air and a small amount of hydrogen through the cathode side of the stack at system shutdown so that the hydrogen and oxygen combine in the cathode side to reduce the amount of oxygen, and thus the potential that causes carbon corrosion.
It is also known to stop the cathode air flow while maintaining positive anode side hydrogen pressure at shutdown, and then to apply a load to the stack to allow the oxygen to be consumed by hydrogen, followed by closing the inlet and outlet valves of the anode and cathode sides. While it has been shown that these techniques do help to mitigate corrosion of the carbon support, these techniques may not remove all of the oxygen, especially from the volumes beyond the stack, or add the complexity of a cathode recycle system. Therefore, there is a need in the art for an improved or simplified way to prevent oxygen rich air from being present at start-up of a fuel cell system.